Allow diffusion to load initial commits in Mercurial repositories

Summary: no parents - no problem - just diff that ish against "null"

Test Plan: initial commits were viewable in my test repos

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1689

Differential Revision: https://secure.phabricator.com/D3660